In digital communication, digital receivers generally require a digital clock synchronized to the received bit stream to control timing of the output of the bit stream. However, the clocks of the transmitter and receiver are generally not synchronous because the transmitter and receiver are separated geographically and because the transmitted signal experiences frequency changes due to noise and other transmission factors.
Products using an I-Q demodulator, such as a modem, need the ability to synchronize received data having widely varying phase. Symbol timing recovery, also known as bit timing recovery, is conventionally performed by extracting the time clock directly from the bit stream and correcting the phase of the extracted clock signal to line up with the demodulator data. According to such systems, the recovered clock signal is adjusted until it is in phase with the received signal.
In a conventional bit timing recovery system, a modulated carrier signal at intermediate frequency is received by a demodulator and converted to baseband. The signal is separated into its inphase and quadrature components, where the inphase branch determines the polarity of the bit transitions and the quadrature channel determines the magnitude of the bit-timing error. An inphase integrator and quadrature integrator adjust the clock of the received signal based on a phase error estimate, and the adjusted clock signal is converted to a digital signal by analog-to-digital converters. The digital inphase and quadrature signals then enter a phase error detector where the polarity and magnitude of the bit-timing error is determined. The phase error estimate is used to control a voltage controlled oscillator which adjusts the received analog clock signal such that it is in phase with the demodulator clock.
The disadvantage of this kind of conventional symbol timing recovery system is that phase error correction is performed on the analog signal and an analog implementation is not capable of handling the range of frequencies that a completely digital system can process. Shifting the phase prior to A/D conversion limits the range of frequencies over which the signal may be tuned. Generally, an analog signal is tuned over a 2 to 1 range in frequency, and beyond that range the voltage controlled oscillator used for phase error correction would have to be changed or additional VCOs having higher frequency ranges added to accommodate the greater frequencies. In order to achieve a wider frequency range more hardware would have to be added to the demodulator, thus making it larger, more complex, and more costly.
The convention symbol timing recovery devices does provide some advantages, namely the ability to receive and process data from a variety of transmitter devices. Nevertheless, the prior art symbol timing recovery devices do not provide the advantage of relatively simple construction in conjunction with the ability to tune over a virtually unlimited frequency range.